Volvo FM Handbook page 148

Slippery surface, no differential lock engaged. Only
the wheel on the slippery surface is rotating, the truck
remains stationary.
Slippery surface, differential lock engaged. The
wheels are forced to turn at the same speed. Only the
wheel on firm ground drives. Accelerate carefully so
that the driving axle and gear are not damaged.
Disengage the differential lock when both wheels are
gripping again.
Engage differential lock
Engage the differential lock directly before coming on
to the slippery surface and disengage the differential
lock when the slippery surface has been passed.
None of the wheels may be spinning before the
differential lock is engaged!
Driving 137
Warning!
Drive carefully when the
differential lock is engaged.
Never turn corners on firm
ground with the differential lock
engaged.
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